Thermoplastic resin films are widely used as the substrate films for use in applications such as packaging of food and for use in general industrial applications. In such applications, the surface of the substrate films is usually subjected to various secondary processings such as coating with various coating materials, printing processing, vapor-deposition processing and laminating processing with other films. For such processings, there have been made attempts in which by applying to the substrate films physical treatments such as corona discharge treatment and plasma treatment or chemical treatments to activate the film surface with chemicals such as acids or alkalis, the surface modification of the substrate films is achieved to enhance the adhesion between the substrate films and the coating substances provided by the above-described secondary processings. However, the methods based on the physical treatments are simple and easy in treatment processes, but the resulting adhesion is insufficient. The methods based on the chemical treatments are complicated in treatment processes, and disadvantageously lead to adverse effects such as the deterioration of the work environment.
Aside from the above-described physical and chemical treatment methods, there is a method in which an easy adhesion coating layer (hereinafter, also referred to as a “primer layer” as the case may be) is formed in a laminated state by coating a substrate film with a primer which has adhesion activity. This method is widely applied because of the fact that this method permits selecting the components of the primer according to different top coat layers (overlayers formed by the secondary processing) and because of other reasons.
As the constituent components of the primer layer, aqueous resins are frequently used from the viewpoints of workability, safety and cost. In particular, resins such as polyurethane resin, polyester resin and acrylic resins are used for the purpose of improving the adhesion to the overlayer such as a coating layer, an ink layer or a vapor-deposition layer.
Additionally, implemented are the improvements of the performances as the primer, in particular, the improvements of the water resistance and the heat resistance by mixing various crosslinking agents with the aqueous resins. Examples of the crosslinking agents used for such purposes include isocyanate compounds and aminoplast resins (JP-56-151562 A). In JP-8-311221 A, presented is the improvement of the water-resistant adhesiveness of a vapor-deposition layer by a primer layer formed of a polyester resin and a melamine resin. In JP-8-332706 A, described is a method for obtaining an easy adhesive laminated film by using a carbodiimide monomer that contains a carbodiimide group per a molecule thereof.
The primer layers obtained by using these existing techniques tend to be degraded in the interlayer adhesion between the substrate film and the top coat layer when subjected to a high temperature hot water treatment such as boiling or retorting, disadvantageously leading to unsatisfactory practical performance as food packaging materials. Examples of the methods developed to improve the adhesiveness after the boiling or retorting treatment include a method in which an aqueous polyester resin having a glass transition temperature of 100° C. or higher and a melamine resin are used to maintain a high adhesiveness even after the retorting treatment (JP-2004-256625 A).